Langmuir 1986, 2, 639-643
639
Adsorption of Surfactant Mixtures in Froth Flotation+ W. von Rybinski and M.J. Schwuger* Laboratories of Henkel KGaA, Dusseldorf, Germany Received February 28,1986. I n Final Form: June 9, 1986 This paper reports on the mode of action of mixtures of anionic and nonionic surfactants in flotation. At special mixing ratios the surface tension is significantly reduced in comparison to that of the anionic surfactant alone. A synergistic behavior is observed. The results are discussed in terms of interaction parameters calculated accqding to the theory of regular solutions. The adsorption of the anionic surfactant on the mineral surface is also influenced by the nonionic surfactant. The results of the physicochemical measurements have been confirmed by flotation tests with a natural scheelite ore. 1. Introduction Froth flotation is one of the key separation procedures in processing of mineral raw materials. The principle of the flotation is based on the fact that hydrophobic particles are concentrated at air bubbles from their aqueous dispersion. From there, they can be separated from the suspension into a layer of froth at the surface of the liquid. Only very few minerals have a hydrophobic surface. Therefore, a most possible selective hydrophobization of the value minerals to be floated and the formation of a buoyant froth in the flotation cell by suitable reagents are decisive issues with respect to a successful separation of mixtures of minerals. In this processing technique various interfacial processes play an important role in the flotation. Surface-active substances serve the formation of froth and the hydrophobization of the surface of the minerals through their adsorption at liquidlgas and liquidlsolid interfaces. Usually, ionic surfactants with different structures are used for the selective hydrophobization of the minerals. They are adsorbed through electrostatic interaction of the polar group of the molecules with ions of the surface of the minerals.’ These reagents are concurrently adsorbed at the waterlair interface. They reduce the surface tension and form buoyant froths. In case of insufficient froth formation by these substances, so-called frothers can be added.2 These frothers are nonionic compounds such as nonpolar oils or alcohol^.^-^ The structure of the froth is modified by the addition of these frothers in such a way that a good secondary concentration takes place in the froth. This term means an enhanced adhesion of the hydrophobized mineral particles at the air bubbles and the reduction of a simultaneous concentration of the hydrophilic particles of the gangue minerals in the froth. The froth in the flotation cell should have a high buoyancy. It should yet rapidly decay after the delivery. So far, little knowledge exists on the mode of action of mixtures of differently structured surfactants in the flotation process. The use of mixtures of anionic collectors with nonionic surfactants with regard to separation problems has been r e p ~ r t e d . ~The $ ~ mechanism of action of these mixtures in flotation processes, their adsorption on minerals, and the formation of froth are, however, still largely unidentified. From other areas, such as the washing process, the positive effect of surfactant mixtures is known and has been used specifically. The mixed adsorption of anionic and nonionic surfactants on carbon black has been describeds and has been transferred to the effect of surfactant mixtures in the washing p r o c e ~ s . ~On the basis of these results, it has been the purpose of our investigat
Presented a t the symposium on “Fluid-Fluid
Interfaces:
Foams”,190th National Meeting of the American Chemical Society, Chicago, IL, Sept 8-13, 1985.
0743-7463/86/2402-0639$01.50/0
tions to elucidate the mechanism of action of surfactant mixtures at the different interfaces in the flotation process with regard to the separation of oxidic ores. 2. Investigated Systems The tests are conducted with the model system scheelitelcalcite because the separation of value minerals from calcite-containing gangue minerals is of great practical importance. Pure scheelite and calcite were selected for the physicochemical measurements. The flotation tests were conducted with a natural scheelite ore with 0.4 wt % tungsten trioxide. The surfactant system consisted of mixtures from an anionic collector with nonionic surfactants. The anionic collector was an alkyl sulfosuccinate (AS) as used in practical flotation. Nonionic surfactants used were nonylphenol polyglycol ether with 5 or 10 ethylene oxide groups in the molecule (NP 5 and NP 10). 3. Results
3.1. Liquid/Gas Interface. A key issue of the flotation is the modification of the liquidlgas interface for the formation of froth. Flotation froths are disperse systems with the three phases solid/liquid/gas. The formation and stability of the flotation froth is affected by a series of parameters: surface tension of the liquid; surface viscosity and elasticity; dimension and polarity of the solid particles embodied in the froth; viscosity of the liquid. Although numerous papers on the mechanism of froth formation and stability have been published so the importance of the individual physicochemicalparameters has been only partially elucidated. If, for better comprehension of froth flotation, the three-phase flotation froth is considered similarly to a two-phase system liquidlgas, physicochemical parameters can be used that exactly (1)Leja, J. Surface Chemistry of Froth Flotation; Plenum Press: New York, 1982. (2)Schubert, H. Aufbereitung fester Mineralischer Rohstoffe; VEB Deubcher Verlag f. Grundstoffindustrie: Leipzig, 1977. (3) Schubert, H.; Schneider, W. Proc. Int. Miner. Process. Congr., 9th 1970. 189. (4) Fuerstenau, D. W.; Yamada, B. J. Trans. Am. Inst. Min. Metall. Pet. Eng. 1962,223, 50. (5)Schulman, J. H.; Leja, J. Kolloid-Z. 1954,136, 107. (6)Doren, A.; van Lierde, A.; de Cuyper, J. A. Dev. Min. Proc., 1979, 2, 86. (7) Giesekke, E. W.; Harris, P. J. S. Afr. J. Chem. 1984,37,96. (8)Schwuger, M.J.; Smolka, H. G. Colloid Polym. Sei. 1977,225,589. (9)Kurzendorfer, C. P.; Schwuger, M. J.; Lange, H. Ber. Bunsenges. Phys. Chem. 1978,82,962. (10)Bikerman, J. J.; Perri, J. M.; Booth, R. B.; Currie, C. C. Foams, Theory and Industrial Applications; Reinhold New York, 1953. (11)Akers, R. J., Ed. Foams; Proceedings Symp. Soc. Chem. Ind.; Academic Press: New York, 1975. (12) Harris, P. J. In Principles of Flotation; King, R. P., Ed.; South African Institute of Mining Metallurgy: Johannesburg, 1982.
0 1986 American Chemical Society
640 Lnngmuir, Vnl. 2, N o . 5 , 1986
uon Rybinski and Schwuger
y mN/m D
60
20
'. '.
-
NPS/AS
-
.-. I-.
\
1/ o 0/1 1/1
- _ _ _ ideal
i/2
-
real
0.3
-,I, A,
0.2 10
10 2
3
10
c
.w,
Figure 1. Surface tension of nonylphenol pentaglycol ether (NP 5 ) and alkyl sulfosuccinate (AS) and their mixtures. T = 23 OC.
characterize the froth properties. A key prerequisite with respect to froth flotation and stability in aqueous systems is the adsorption of surface-active substances at the liquid/gas interface. This effect can be characterized by the measurement of the surface tension. For certain classes of compounds there exists a direct correlation between the reduction of the surface tension and the intensity of froth formation.13J4 Figure 1shows the surface tension of alkyl sulfosuccinate (AS) and nonylphenol pentaglycol ether (NP 5 ) and their mixtures vs. the concentration of the solution. From this graph a critical micelle concentration (cmc) of 3.5 X lo-' g/L or 6.0 X lo4 mol/L can be read for the alkyl sulfosuccinate. The corresponding values for the nonionic surfactant NP 5 are 1.4 X g/L and 3.2 X mol/L. The critical micelle concentration of the nonionic surfactant is thus 10 times smaller than that of the anionic surfactant. For mixtures of surfactants with different molar ratio the measured curves are significantly closer to the values of the nonionic surfactant even with a high proportion of anionic surfactant. This indicates a synergistic behavior of the surfactants in the mixtures. The interaction between two surfactants has been calculated by Rosen et al.15 according to the theory of the regular solutions.16 According to these calculations, predictions with regard to a synergistic behavior of the surfactants can be made from the change of the critical micelle concentration. Basically, two individually different cases can be considered. Ideal Behavior. The critical micelle concentration c,d of the mixtures is described by the following equation:
0.1
* 0.5
0
&NPS
1.0
Figure 2. Comparison of the critical micelle concentrations of alkyl sulfosuccinate (AS) and nonylphenol pentaglycol ether (NP 5) mixtures with the ideal mixture model.
pulsive interactions forces and fi < 1 for attracting interactions. The activity coefficients cannot be measured experimentally. However, they can be calculated with the help of the model for regular solutions17from experimental data. The following relations can be made with respect to the activity coefficient: fl
= exp(P)(1- xJ2
(3)
f2 = exp(P)x12
(4)
where x1 = mole fraction of the component 1 in the mixed micelle. The interaction parameter /3 that must be negative in case of a synergistic behavior of the surfactant is related to the micelle formation enthalpy AH, by the following equation: AHm = PRTx,(l - X I ) (5) where R = gas constant and T = temperature. In order to calculate 0from the obtained data one can put up the following equations according to the phaseseparation model:16
By means of the Duhem-Margules equation where c1 = cmc of component 1, c2 = cmc of component 2, and al = mole fraction of the component 1 in the mixtures. Real Behavior. The critical micelle concentration of the mixtures cr, is given by
and from the equations (6) and (7) one obtains
where fl = activity coefficient of the component 1 in the mixed micelle and f i = activity coefficient of the component 2 in the mixed micelle. Either repulsive and attracting interactions may occur, whereby f , > 1 for re-
This equation can be solved iteratively and serves the calculation of P according to
(13) Posner, A. H.; Anderson, J. R.; Alexander, A. E. J. Colloid Sci. 1952. 7. 623. ----I
- I
(14) Dudenkov, S. V.; Shafeev, E. S. Tsuetn. Met. (Moscow) 1966,39, 12. (15) Rosen, M. J. J . Colloid Interface Sci. 1982, 86, 164. (16) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979.
(17) Hildebrandt, J. H.; Scott, R. L. Regular Solution; Prentice-Hall: Englewood Cliffs, NJ, 1962.
Langmuir, Vol. 2, No. 5, 1986 641
Adsorption of Surfactant Mixtures
A-A
foam height [cm]
-2
0-0
AS NP10 NP5
5
Figure 4. Adsorption of anionic and nonionic surfactants on
scheelite.
I I
0
20
40
60
100 weightohofNP5
80
/
:j I
Figure 3. Foam height of mixtures from alkyl sulfosuccinak (AS) and nonylphenol pentaglycol ether (NP 5 ) after 30 s.
With known cmc of the individual components and of one mixture, the cmc vs. the composition of the mixtures can be deduced from this equation. The results of this calculation is shown in Figure 2 for the investigated system alkyl sulfosuccinate/nonylphenol pentaglycol ether. It is also compared with the ideal behavior of the mixture. The calculated curve was confirmed by experimental points and shows a synergistic behavior of the surfactants in that a reduction of the cmc is observed when small amounts of NP 5 are added. The calculated interaction parameter 6 is -3.6 and indicates attractive interaction forces between the surfactants in the mixed micelle. When increased interactions and synergistic behavior in micelles are observed, it can be analogously assumed that increased interactions also exist at the water/air interface. This should also be shown by the formation of froth. For this purpose the froth height of the surfactant solutions vs. mixing ratio was measured under standardized conditions after 30 s. The results of these tests are shown in Figure 3. The foam formation of the mixture is stronger than that of the individual surfactants above a distinct total concentration of the surfactants. The maximum of the foam formation occurs when N P 5 is added in approximately 30-40 wt 70 and appears to be largely independent of the total concentration of the surfactants. Above this maximum the foam formation decreases almost linearly with increasing concentration of nonionic surfactants. As a consequence of the increased interactions between the surfactants, a positive impact of the surfactant mixtures is shown with respect to the foam formation in comparison with the individual surfactants. 3.2. Liquid/Solid Interface. The selective adsorption of the surfactants on the surface of the minerals is the second critical parameter of the flotation. Secondary reactions have to be considered in addition to the actual adsorption with respect to minerals such as scheelite and calcite and the use of anionic collectors. These secondary reactions can affect the efficiency of the flotation process. The cause of this lies in the solubility of the minerals in water.ls Multivalent cations dissolved from the minerals may react with anionic surfactants and form precipi~
(18)Atademir, M.R.; Kitchener, J. A.; Shergold, H. L. J . Colloid Interface Sci. 1979, 71, 466.
-
-
0
.
c.10~
0
1
2
3
mole/^]
Figure 5. Adsorption of anionic and nonionic surfactants on
calcite.
t a t e ~ . ' The ~ ~ ~precipitation ~ reactions affect the hydrophobization of the mineral surfaces by the adsorption of surfactants. This has been proved by calorimetric measurements21 The usual method for determination of adsorption isotherms from liquids is the measurement of the concentrations in the solution before and after establishment of the adsorption equilibrium. The adsorbed amounts result from the difference between these two quantities. If precipitation reactions occur in addition to adsorption, the precipitation products are also measured. The quantity called "adsorbed amount" characterizes the sum of adsorption and precipitation in case of an investigation of partly soluble minerals. The concentrations of the dissolved anionic surfactants were determined by potentiometric titration. The concentration of the nonylphenol polyglycol ethers was measured by UV spectroscopy. Figure 4 shows the result of these measurements with scheelite. It is seen that the adsorption isotherm of the anionic surfactant is formally of the Langmuir type. The isotherm attains a plateau at higher concentrations of the solution. The nonionic surfactants are significantly less adsorbed and show a different adsorption behavior. In the explored concentration range, the adsorbed amounts are almost linearly dependent on the equilibrium concentration. The adsorption on the mineral surface decreases with increasing number of polyglycol groups of the nonionic surfactant. This effect can be explained by an increasing hydrophilicity and a larger area per surfactant molecule at the interface. The reason for the small adsorption of the nonionic surfactant on the polar surface of the scheelite probably is the lack of electrostatic interactions between adsorbent and adsorbate. ____
~
~~~~
(19) du Rietz, L. Proc. Int. Miner. Process. Congr., 4th 1957, 417. (20) Somasundaran, P. J. Colloid Interface Sci. 1966, 31, 557. (21) von Rybinski, W.; Schwuger, M. J. Ber. Bunsenges. Phys. Chem. 1984,88, 1148.
642 Lnngmuir, Vol. 2, No. 5, 1986
von Rybinski and Schwuger
Table I. Flotation of a Scheelite Ore with Alkyl Sulfosuccinate (AS) and Mixtures from AS and Nonylphenol Pentaglycol Ether (NP 5 ) in a Microflotation Cella
concentrate grade, o/o
total,
NP 5, g / t
AS, g l t
RWO33
3.9
500 250 150 100
52 39 51 55
8.7 4.9 3.8
250 150 100
%
wo3
CaO
SiOz
A1208
Fez03
0.4 5.3 1.8 4.1 5.9
6.8 19.1 11.5 15.6 15.8
59.5 36.2 47.7 41.0 39.6
12.1 11.6 12.5 11.8 10.2
7.0 8.6 8.2 9.4 9.5
pH value 8.5-9.0 and flotation time 2 min.
r . lo6 [mole/m']
AS/NP5 >.-
